Re: [PATCH v2 1/1] iommu/vt-d: Serialize IOMMU GCMD register modifications

From: Lu Baolu
Date: Thu Aug 27 2020 - 20:05:42 EST


Hi,

On 8/27/20 1:39 PM, Tian, Kevin wrote:
From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Sent: Thursday, August 27, 2020 12:25 PM

The VT-d spec requires (10.4.4 Global Command Register, GCMD_REG
General
Description) that:

If multiple control fields in this register need to be modified, software
must serialize the modifications through multiple writes to this register.

However, in irq_remapping.c, modifications of IRE and CFI are done in one
write. We need to do two separate writes with STS checking after each.

Fixes: af8d102f999a4 ("x86/intel/irq_remapping: Clean up x2apic opt-out
security warning mess")
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
---
drivers/iommu/intel/irq_remapping.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

Change log:
v1->v2:
- v1 posted here
https://lore.kernel.org/linux-iommu/20200826025825.2322-1-
baolu.lu@xxxxxxxxxxxxxxx/;
- Add status check before disabling CFI. (Kevin)

diff --git a/drivers/iommu/intel/irq_remapping.c
b/drivers/iommu/intel/irq_remapping.c
index 9564d23d094f..7552bb7e92c8 100644
--- a/drivers/iommu/intel/irq_remapping.c
+++ b/drivers/iommu/intel/irq_remapping.c
@@ -507,12 +507,19 @@ static void iommu_enable_irq_remapping(struct
intel_iommu *iommu)

/* Enable interrupt-remapping */
iommu->gcmd |= DMA_GCMD_IRE;
- iommu->gcmd &= ~DMA_GCMD_CFI; /* Block compatibility-format
MSIs */
writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
-
IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
readl, (sts & DMA_GSTS_IRES), sts);

+ /* Block compatibility-format MSIs */
+ sts = readl(iommu->reg + DMAR_GSTS_REG);

no need of this readl as the status is already three in IOMMU_WAIT_OP.

Yes. Really!

Best regards,
baolu


+ if (sts & DMA_GSTS_CFIS) {
+ iommu->gcmd &= ~DMA_GCMD_CFI;
+ writel(iommu->gcmd, iommu->reg + DMAR_GCMD_REG);
+ IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG,
+ readl, !(sts & DMA_GSTS_CFIS), sts);
+ }
+
/*
* With CFI clear in the Global Command register, we should be
* protected from dangerous (i.e. compatibility) interrupts
--
2.17.1